(1) Field of the Invention
The present invention relates to an image forming apparatus such as a copier, printer etc., for performing electrophotographic image formation, in particular, relating to an image forming apparatus having a toner recycling function of reusing the toner collected from the photoreceptor surface.
(2) Description of the Prior Art
In the image forming process using the electrophotographic technique effected in image forming apparatus such as copiers, printers and the like, a static latent image is formed on the photoreceptor surface by the photoconductive effect and developed into a visual image with a powder developer supplied from a developing device. This visual image supported on the photoreceptor is transferred to a recording medium. At that event, some part of the developer constituting the visual image on the photoreceptor surface will remain on the photoreceptor surface, failing to transfer to the recording medium. In general the photoreceptor surface is used repeatedly for image forming operations, so if the remaining developer is left as it is on the photoreceptor surface, this will cause degradation of the image quality for subsequent image forming operations. Therefore, the developer left on the photoreceptor surface after the transfer step is removed by means of a cleaning device and discarded.
If all the developer not having been used for image forming on the recording medium during the image forming operation is to be discarded, the developer is wasted, raising the running cost.
A conventional image forming apparatus using a dual-component developer consisting of a toner and a carrier has a so-called toner recycling function for collecting the toner which has been removed from the photoreceptor surface by a cleaning device, into the developing device, and reusing the toner thus removed from the photoreceptor surface for subsequent image forming, to thereby avoid waste of toner and realize a reduction in running cost.
Typically, as shown in FIG. 1, provided around and opposing a photoreceptor drum 51 are a primary charger 52, a developing unit 54, a transfer device 55, a cleaning device 57 and a charge erasing device 58, in this sequential order, and furthermore, a conveying device 59 for conveying toner from the cleaning device 57 to the developing unit 54. Cleaning device 57 located between transfer device 55 and primary charger 52 with respect to the rotational direction of photoreceptor drum 51 includes a scraper member 57a such as a blade etc., abutting the surface of photoreceptor drum 51 and collects the toner remaining on photoreceptor drum 51 after passage of the opposing position of transfer device 55 and falling into cleaning device 57. The toner collected in cleaning device 57 is conveyed to developing unit 54 by driving at an appropriate timing, a conveying device 59 constituted by, for example, a screw conveyer mechanism.
In order to maintain a constant density of the image formed on the recording medium in the image forming operation by the electrophotographic technique using a dual-component developer, a necessary and sufficient amount of toner for developing the static latent image needs to be stored in the developing unit. Therefore, a conventional image forming apparatus, for example, includes a toner concentration sensor such as a magnetic permeameter or the like which detects the amount of the toner stored in the developing unit based on the detection signal from the toner concentration sensor and replenishes the developing unit with the toner based on the detection result. Since only the toner among the developer is consumed during the image forming process while the stored amount of the carrier in the developing unit will be theoretically unchanged, variation in the ratio of the toner to the developer consisting of the magnetic component, i.e., the carrier and the non-magnetic toner will appear as the change in magnetic permeability of the developer. Accordingly, the amount of the toner stored in the developing unit can be obtained from the detection result of the permeability of the developer by the magnetic permeability sensor.
However, so-called recycled toner, which has been removed from the photoreceptor surface after the transfer step and collected into the developing unit contains foreign substances such as paper dust, talc etc. from the paper as the recording medium and this recycled toner is returned into the developing unit, so that the foreign substances mixed with the developer have an influence on the detection result of the toner concentration sensor. However, there has been no conventional image forming apparatus having a toner recycling function, which takes into consideration the variation of the detection result of the toner concentration sensor due to the foreign substances mixed with the developer in the developing unit. Thus, it has been impossible to detect an exact amount of the toner stored in the developing unit, which might cause surplus or deficiency of the amount of the toner replenishing the developing unit, making it impossible to maintain correct image density.
Further, the amount of foreign substances contained in the recycled toner will vary depending upon image forming conditions such as the number of image forming operations, the sizes of recording media as well as environmental conditions such as temperature, humidity etc. Therefore, it has been impossible to definitely determine the amount of foreign substances mixed in the developer based on the amount of the recycled toner collected into the developing unit, and it has not been easy to compensate the errors arising in the detection result of the toner concentration due to mixture of foreign substances in the developing unit. Hence, it has been difficult for an image forming apparatus having a toner recycling function to keep a correct image density.